Engine systems may utilize one or more gas constituent sensors, such as oxygen sensors, for sensing an oxygen concentration of air flowing through engine air passages. In one example, an engine system may include one or more intake oxygen sensors (IAO2) located in the engine intake. For example, an intake oxygen sensor may be positioned in an intake passage, downstream from a compressor and a charge air cooler, to provide an indication of EGR flow. In another example, the engine system may include one or more exhaust gas sensors in an exhaust system of the engine system to detect an air-fuel ratio of exhaust gas exhausted from the engine. Under certain engine operating conditions, such as a cold start or condensate formation, water may splash against and contact the oxygen sensor. When water contacts the oxygen sensor, the temperature of the sensor begins to decrease. As a result, heater power of a heating element of the oxygen sensor increases to increase the sensor temperature. When the heater power increases for an extended period of time when water is on the oxygen sensor, the heating element may crack, thereby degrading the oxygen sensor.
Other attempts to address degradation of the oxygen sensor due to water contacting the sensor include adjusting a heating power of a heating element of the oxygen sensor. One example approach is shown by Surnilla et. al. in US 2015/0076134. Therein, the heater power of the heating element of an oxygen sensor is adjusted in response to an increase in the heater power by a threshold amount. As an example, a baseline power level of the oxygen sensor is determined during a condition wherein no water is contacting the sensor. When heater power increases above the baseline power level (e.g., due to water splashing on the sensor), the heating power may be decreased by turning off power of a heating element of the sensor. After a certain duration, the heater power may be turned back on and increased to baseline power level. In this way, reducing the heater power when water is indicated at the oxygen sensor may reduce oxygen sensor degradation via cracking of the heating element.
However, the inventors herein have recognized potential issues with such systems. For example, turning off the heater power to the heating element when water is indicated on the sensor, and then turning the heater back on after a certain time leads to fluctuations in the temperature of the sensor. As such, the fluctuations in the sensor temperature may adversely affect the ability of the sensor to sense oxygen concentration in the exhaust. In addition, the output of the sensor during such conditions may be inaccurate, thereby affecting air fuel control and hence affecting the performance of the engine.
In one example, the issues described above may be addressed by a method for an engine method, comprising indicating water at an exhaust oxygen sensor positioned in an engine exhaust passage based on a sensor parameter of the exhaust oxygen sensor while operating the exhaust oxygen sensor in a variable voltage (VVs) mode where a reference voltage is adjusted from a lower, first voltage to a higher, second voltage, and adjusting one or more of sensor operation and engine operation based on the indicating water. In this way, sensor degradation may be reduced.
As an example, the exhaust oxygen sensor may be traditionally operated in a non-VVs mode wherein the sensor is operated at the lower voltage, and an output of the exhaust oxygen sensor may be used for determining an air-fuel-ratio (AFR). As such, water at the exhaust oxygen sensor may be detected based on the sensor parameter such as a pumping current or change in pumping current of the exhaust oxygen sensor. For example, when the pumping current of the sensor falls below a threshold current, water splash on the sensor may be indicated. In another example, if the change in pumping current is higher than a baseline change in pumping current (e.g., when no water is present on the sensor), then water splash on the sensor may be indicated. When water splash on the sensor is indicated, the exhaust oxygen sensor may no longer be used as a traditional air-fuel sensor; instead, the sensor may be transitioned from the non-VVs mode to the VVs mode, where the sensor is operated at the higher voltage and/or modulated between the lower voltage and higher voltage. While the exhaust oxygen sensor is not operated as the traditional air-fuel sensor, AFR may be estimated using a different, downstream sensor, and/or using previously determined AFR. In this way, air fuel control may not be affected while the exhaust oxygen sensor is being operated in the VVs mode.
Additionally, the sensor parameters may be checked continuously while the sensor is in VVs mode to determine when water has evaporated from the exhaust oxygen sensor. For example, once the change in pumping current reaches the baseline change in pumping current, then the exhaust oxygen sensor is considered “dry”, and the sensor may then be transitioned back to the non-VVs mode. Once the exhaust oxygen sensor is in the non-VVs mode, the sensor may be used to determine AFR. In this way, sensor degradation may be reduced, and the integrity of the exhaust oxygen sensor may be maintained. Further, an accuracy of AFR estimates based on the exhaust oxygen sensor output may be increased, thereby increasing engine efficiency.
It should be understood that the summary above is provided to introduce in simplified form a selection of concepts that are further described in the detailed description. It is not meant to identify key or essential features of the claimed subject matter, the scope of which is defined uniquely by the claims that follow the detailed description. Furthermore, the claimed subject matter is not limited to implementations that solve any disadvantages noted above or in any part of this disclosure.